The present invention relates generally to the field of image formation and in particular to an automatic media alignment nip release mechanism.
Media sheets in image formation devices are preferably aligned to eliminate skew. That is, the leading edge of the media sheet is aligned to be parallel to the axes of the photoconductive members used to transfer developed images to the media sheet. A media sheet is aligned if, when crossing a line across the media path perpendicular to direction of travel, the leading edge of the media sheet encounters the line at the same time along its extent. A media sheet is skewed if, for example, when crossing such a line, one of the leading corners of the media sheet encounters the line before the other leading corner.
One method of media sheet alignment known in the art is referred to herein as “bump alignment.” A media sheet is directed to the nip formed between two rollers, the surfaces of which are in contact along their length. The media sheet is briefly driven after the leading edge has contacted the nip of rollers, causing a “bump” or curvature, to form in the media sheet. This deformation of the media sheet exerts a forward force on the leading edge of the sheet, forcing it against the nip of the rollers. The malleability of the deformation allows either leading corner to advance slightly, relative to and independently of the other leading corner, until the leading edge is flush with the nip along its entire length, thus eliminating any skew in the positioning of the leading edge relative to the axes of the two rollers. At least one of the rollers is then driven, capturing the leading edge of the media sheet simultaneously along the nip, and passing the media sheet along in an aligned orientation.
In some cases, a registration function may be combined with the media sheet alignment function. Registration refers to the timing of advancing a media sheet into the image formation path. Advancing a media sheet from a known position (the nip of the alignment/registration rollers) at a specific time provides a precise temporal demarcation against which downstream image formation processes may be referenced, to ensure high quality image formation (such as, for example, ensuring accurate registration of plural color planes transferred at plural image formation stations).
To function properly, the alignment nip must be formed with a relatively high nip force. This presents a difficulty in clearing jams, as the alignment rollers grip the media sheet tightly. Additionally, in image forming devices wherein parts of the media sheet travel path separate, such as when a door or other subunit on which some elements are mounted is opened or otherwise moved away from the main housing, damage may result if a media sheet spans the separation point, and is being tightly held in the alignment nip.